1. Field of the Invention
The present invention relates to the field of digital transmission and more specifically to an apparatus and method for timing recovery in the reception of base band digital signals.
2. Description of the Prior Art
One of the main problems in data receiving and transmitting devices, such as modems, is the correct reconstruction of the received timing signals, whatever the transmission channel adopted (coaxial cable, optical fiber, radio link, etc.).
In particular, it is desired to reconstruct and use the received timing signal downstream of the demodulator assuming that on the transmission channel there is only the data signal and not the timing signal.
The received timing signal is reconstructed and serves to sample at regular time intervals and hence read the received data signal correctly.
For some kinds of modulation, e.g. multilevel QAM modulation (16-QAM, 64-QAM, 256-QAM) and always where there is strong intersymbol interference due to dispersion phenomena on the transmission channel, e.g. selective fading, for correct interpretation of the received signal, positioning of the sampling instant inside the associated interval becomes critical.
Various timing recovery devices are presently known in the reception base band of digital signals. The use of such known devices becomes problematic when it is necessary to position with extreme accuracy and stability the instant of sampling of the signal received because of the following main shortcomings:
drift of the optimal sampling instant for thermal or ageing reasons, found particularly in devices which require setting of the timing signal phase;
volatility of the control signal of the timing signal phase, encountered in all devices which control the phase automatically with certain algorithms, because the control signal often has an intrinsic noise which creates an unacceptable jitter on the sampling instant;
insufficient accuracy of the control signal due to the fact that the position of the sampling instant is optimal in the presence only of thermal noise on the transmission channel or not in the presence of selective fading.
An example of a known timing recovery system is described in the article by L. E. Franks, G. B. Bubrousky entitled "Statistical properties of timing jitter in a PAM timing recovery scheme", IEEE Transactions on Communications, Vol. COM-22, pages 913-920, Jul. 14, 1974.
This known system extracts a clock-frequency line from the spectrum of the received data signal, either in the intermediate frequency or in the base band, by a nonlinear operation and subsequently filters said line with a narrow band filtering. This system is still much used today in the practical embodiment of QAM demodulators and is often preferred to more recent systems for its simplicity, operating safety, which depends neither on the reconstruction loop of the carrier nor on the equalization control loop in base band, and for fair behavior in the presence of selective fading.
Unfortunately this known system has shortcomings, the most serious of which is drift, either thermal or due to ageing, of the optimal sampling instant.
More recent known sampling systems display uncertain behavior in the presence of selective fading. This is due mainly to crumbling of the sensitivity of the so-called S-curve (defined below and the behavior of which indicates the value of the timing recovery algorithm) with increase of intrinsic noise and precariousness of convergence of the algorithm which controls the timing signal phase. Zero-crossings of the S curve in undesired timing positions are also encountered particularly at pejorative values of the mean square error. Ordinarily these systems use a single signal sample per symbol time and do not allow extraction of sufficient information for reconstruction of the timing signal.
There was recently proposed a timing recovery device of the sampling type using two signal samples per symbol time and described in the article by F. M. Gardner "A BPSK/QPSK timing error detector for sampled receivers", IEEE Transactions on Communications, Vol. COM-34, pages 423-429, May 14, 1986. The algorithm made with this device makes timing recovery independent of the carrier phase and hence of the reconstruction loop thereof but still has the shortcoming that the sensitivity of the S-curve at the sampling instant is not very high. In particular under conditions of selective fading the zero-crossing points of the S-curve do not coincide with the minimum points of the mean square error, obtaining higher error probability.